While the search for a vaccine and a cure for HIV-1 are ongoing, new prevention strategies are needed. Two possibilities are microbicides and pre-exposure prophylaxis. Both have shown only partial efficacy in clinical trials. The basis for these results, in particular for the CAPRISA 004 trial, are uncertain but likely include poor adherence. Based on our recent results, one additional possibility could be a lack of protection from cell-associated transmission. Specifically, vaginal exposure of humanized mice with HIV-1 infected cells resulted in successful infection of 9/10 mice. Having established this cell-associated HIV-1 transmission model, we next replicated the CAPRISA 004 experimental approach and determined if vaginal administration of anti-HIV drug tenofovir inhibits cell-associated HIV-1 infection. Surprisingly, we found tenofovir treatment resulted in only 33% protection against cell-associated HIV-1. This highly contrasts with our observation with cell-free HIV-1 which demonstrated 88% protection with tenofovir. These striking results have significant implications for the development of preventative approaches since they suggest that methods that effectively prevent cell-free HIV-1 transmission may not prevent cell-associated HIV-1 transmission. Therefore, it is of great importance to implement and validate in vivo models of cell-associated HIV-1 transmission that can be used to identify drug strategies that are effective at preventing both modes of HIV-1 infection. Based on this compelling preliminary data, the fundamental hypothesis that will be tested is that cell-associated HIV-1 transmission is more efficient than cell-free virus and is less susceptible to microbicides due to the ability to migrate away from the vaginal vault. This hypothesis will be tested by determining the relative efficacy of transmission by cell-free and cell-associated HIV-1. Microbicides using different anti-viral drugs that target different steps of the HIV-1 life cycle will also be evaluated for the ability to prevent cell-associated HIV-1 transmission. Finally, we will evaluate pre-exposure prophylaxis (PrEP) based strategies for their ability to determine if they can block cell-associated HIV as well. By determining which drugs and administration methods are effective, we can gain insight on the potential mechanisms of cell-associated HIV-1 transmission. The results of these studies will likely influence the future design of HIV-1 prevention strategies. PUBLIC HEALTH RELEVANCE: HIV-1 continues to be a global problem and we are lacking effective strategies to prevent its spread. We believe that cell-associated HIV contributes to HIV transmission in humans. This study proposes to establish the role of cell-associated HIV-1 transmission in vivo using the BLT humanized mice model. The results of these studies have the potential to great impact the design of future HIV-1 prevention strategies that would lead to decreased rates of HIV transmission and improved global health.